Topband: 4 square receiving array
k7tjr at pacifier.com
Tue Jan 30 21:34:05 EST 2007
Using transformers to extract the signals from a short antenna will work but using them in a 4 square would depend on how accurate you would really like to be in your phasing. There are two issues. Winding a 16 to 1 transformer for RF is a marginal thing to begin with due to the winding capacitances involved. Secondly, more important than that is the magnetizing inductance of the 1200 ohm side of the transformer. Normally one would strive for inducive reacance of 10 times the operating load or 12000 ohms in this case when designing a transformer. If you look at the phase shift of the resulting voltage when fed with short antenna you will find almost 90 degrees (almost resonant ) which in itself is not all that bad if all antennas shift the same. This happens even with the 1200 ohm resistive load reflected from the primary. The problem is that core material properties vary widely in their single turn inductance values which in turn from transformer to transformer would be highly unpredictable. Especially when fed with an antenna impedance that might vary a few percent unpredictably (you are only dealing with 75 pf at 160 meters). The real phase shift would be a lot easier to control with a loaded and tuned element (setting the phase) or having the antenna fed into a high impedance amplifier ( mostly capacitive input impedance). I did a quick Spice model of the circuit and changing the magnetizing inductance by as little as 10 percent varied the phase by 7 degrees.
Lee K7TJR Oregon
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